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It looks important to know what species of oak this was associated with. Apparently different species may appear at different locations on the tree. The only place I find oak galls on Quercus garryana are the leaves, and usually they remain attached to the leaf even after it has fallen.

Specimen does appear to be an oak leaf gall, an organic blister produced by an oak in response to a sting by certain insects on the leaf, causing the cells to enlarge and form this mostly hollow living quarters away from predating eyes. If I am correct, the gall should have been attached at some point to an actual oak leaf, if it is not currently attached. Examination of oak leaves still on trees should yield other galls nearby.

I couldn’t say. I didn’t know what galls are untill just now after searching the net.
I find these often. The colors fade over time and they eventually burst and expose a hollow inside. I assumed that this is the method for dispersing spores.